The present invention relates to a method for braking a vehicle, wherein the vehicle comprises a circuit adapted for transmitting a brake signal from an operator controlled braking element to brake devices arranged at a plurality of the vehicle's ground engaging elements via a brake fluid. The invention also relates to a corresponding system.
The invention is especially directed to a system where the operator controlled braking element (for example a foot pedal) upon depression acts on a valve and thereby directly controls the brake devices via the fluid. Work machines, such as dump trucks and especially articulated haulers, have such systems, wherein the brake devices (wheel brakes) are operated via a hydraulic circuit.
The invention will below be described for an articulated hauler. This should however be regarded as a non-limiting example, wherein the invention may be realized in other types of work machines, such as wheel loaders, or other types of vehicles, such as trucks.
An articulated hauler is defined by a pivot joint arranged to allow a front frame and a rear frame to pivot in relation to each other around an imaginary longitudinal axis, that is an axis that runs in the longitudinal direction of the vehicle. Travelling on uneven ground is substantially improved by virtue of such a joint. Further, an articulated hauler normally has six wheels. A front wheel axle is arranged at the front frame and a pair of bogie axles is arranged at the rear frame.
In connection with transportation of heavy loads, e.g. in contracting work, a work machine of the type of an articulated hauler is frequently used. Such vehicles may be operated with large and heavy loads in areas where there are no roads, for example for transports in connection with road or tunnel building, sand pits, mines and similar environments.
The articulated hauler has several brake functions. A primary brake function is achieved via wheel brakes. One brake device is arranged for braking an individual wheel. The wheel brakes are preferably formed by disc brakes. A secondary brake function is achieved via one or several auxiliary brakes to assist the wheel brakes with controlling or decelerating the vehicle. More specifically, the auxiliary brake is adapted to brake the rotational speed of an element in the vehicle's powertrain.
The auxiliary brake may comprise an engine brake and/or a transmission brake (which is called a retarder in an automatic gearbox). The engine brake may be formed by a compression and/or exhaust brake integrated into the engine. The transmission retarder may be formed by a hydraulic retarder that acts inside the transmission. The retarder may be formed by a hydraulic or flow brake for braking a transmission output against a relatively fixed or stationary part. The engagement and disengagement of the retarder can take place by filling and emptying of the hydraulic brake. There is also a parking brake function in an articulated hauler, which is adapted for preventing the hauler from moving from a standstill.
In a known articulated hauler, a brake pedal is not only arranged for controlling the wheel brakes but also for controlling the retarder function. More specifically, only the retarder function is activated when the brake pedal movement is increased from zero or rest position to a specific, intermediate point in its movement range. More specifically, an electric sensor is adapted to sense when the brake pedal is deflected and the retarder function is applied to exert full brake power responsively. The wheel brakes are activated upon further pedal deflection past the specific intermediate point and act together with the retarder function in that subsequent pedal deflection range. More specifically, the foot pedal acts directly on a valve in a hydraulic circuit, which is adapted to control the wheel brakes, in the subsequent pedal deflection range
It is desirable to achieve a method which creates conditions for a variable and safe braking in a cost-efficient way.
A method according to an aspect of the present invention comprises detecting a fluid pressure in the circuit, using the detected fluid pressure level as an input for determining a brake power for at least one auxiliary brake in the vehicle, and controlling the auxiliary brake responsively.
Thus, brake power is achieved directly via the wheel brakes upon deflection of the brake pedal. In addition to the wheel brake power, brake power can be apportioned variably to the auxiliary brake (s). The invention creates further conditions for adjusting the amount of brake power to each individual auxiliary brake depending on the detected fluid pressure level.
Further, detecting a fluid pressure level in the wheel brake circuit is a cost-efficient way of achieving a reliable input signal for determining a brake power for the auxiliary brake.
According to a preferred embodiment, the method comprises the step of determining a relationship between the brake power of the brake devices at the ground engaging elements and the brake power of the auxiliary brake. In other words, the brake power is apportioned in a certain way as a function of the detected pressure in the wheel brake circuit. Specifically, the brake power of the auxiliary brake is calculated based on the detected pressure in the wheel brake circuit. Especially, the method comprises the step of determining a higher brake power portion of the auxiliary brake at a higher circuit pressure.
According to a preferred embodiment, the method comprises the step of determining the auxiliary brake power in response to an available cooling power of the brake devices. This step creates conditions for applying a relatively high brake power via the auxiliary brake and thereby reducing wear on the wheel brakes in certain operating conditions.
According to a further preferred embodiment, the method comprises the step of detecting at least one vehicle operational parameter and using the operational parameter signal as a further input for determining the brake power for said at least one auxiliary brake. In other words, the relative proportions of the brake power of the wheel brakes and the auxiliary brake are altered for different conditions. Of course, also the relative proportions of the brake power to each of the auxiliary brakes may be altered for different conditions.
The operational parameter may for example indicate the vehicle speed, a current gear or a slip of one of the ground engaging element. Accordingly, a higher brake power portion of the auxiliary brake is determined for a higher speed and/or a higher gear and a smaller brake power portion of the auxiliary brake is determined at a higher slip.
According to a specific example, the auxiliary brake is controlled to exert full power already at low detected wheel brake pressure levels if the vehicle travels at high speed. On the other hand, at low vehicle speeds, the wheel brake pressure level has to be very high in order for the auxiliary brake to exert full power.
It is desirable to achieve a system which creates conditions for a variable and safe braking in a cost-efficient way.
A system for braking a vehicle according to an aspect of the present invention comprises a circuit adapted for transmitting a brake signal from an operator controlled braking element to brake devices arranged at a plurality of the vehicle's ground engaging elements via a brake fluid and means for detecting a fluid pressure in the circuit, characterized in that the system comprises means for determining a brake power for at least one auxiliary brake in the vehicle on the basis of the detected fluid pressure level, and means for controlling the auxiliary brake responsively.
Other advantageous features and functions of various embodiments of the invention are set forth in the following description and in the dependent claims.